1. Field of the Invention
The invention relates to a magnet module for a nuclear magnetic flow meter with an arrangement which comprises at least one permanent magnet as a component.
2. Description of Related Art
Nuclear magnetic flow meters determine the flow rate of the individual phases of a multiphase fluid, the flow velocities of the individual phases, and the relative proportions of the individual phases in the multiphase fluid in a measurement tube by measuring and evaluating the voltage induced by nuclear magnetic resonance of the multiphase fluid into a suitable sensor. The measurement principle of nuclear magnetic resonance is based on the property of atomic nuclei with a free magnetic moment to precess to the nuclear spin in the presence of a magnetic field. The precession of a vector representing the magnetic moment of an atomic nucleus takes place around a vector representing the magnetic field in place of the atomic nucleus. The precession induces a voltage into the sensor. The frequency of the precession is called the Larmor frequency ωL and is computed according to ωL=γ·B, γ being the gyromagnetic ratio and B being the amount of the magnetic field strength. The gyromagnetic ratio γ is maximum for hydrogen nuclei. For this reason, fluids with hydrogen nuclei are especially suited for nuclear magnetic flow meters.
A multiphase fluid consisting essentially of crude oil, natural gas, and salt water is delivered from an oil source. So-called test separators branch off a small part of the delivered fluid, separate the individual phases of the fluid from one another, and determine the proportions of the individual phases in the fluid. Test separators, however, are expensive, cannot be installed under the sea, and do not allow real-time measurements. In particular test, separators are unable to reliably measure crude oil proportions smaller than 5%. Since the crude oil proportion of each source drops continuously and the crude oil proportion of many sources is already less than 5%, it is currently impossible to exploit these sources in an economically efficient manner.
Both crude oil and also natural gas and salt water contain hydrogen nuclei, for which, as already mentioned, the gyromagnetic ratio γ is maximum. Nuclear magnetic flow meters are therefore suited especially for use on oil sources, and for use undersea directly on a source on the sea bed; but are not limited to these applications. Other applications arise, for example, in the petrochemical or in the chemical industry. Branching off a part of the fluid is not necessary, rather the entire fluid is measured in real time. Compared to test separators, nuclear magnetic flow meters are more economical and require less maintenance and can also especially reliably measure crude oil proportions less than 5% in the fluid, as a result of which the further exploitation of a host of oil sources becomes possible for the first time.
U.S. Pat. No. 7,872,474, discloses a magnetization device including a stack of disk magnets which forms a hollow cylindrical permanent magnet. The magnetic field is homogeneous in the cylindrical interior of the magnetization device. The disk magnets in the stack are fixed by screws of a nonmagnetic material. Each of the disk magnets includes magnet modules, wherein each of the magnet modules consists of a rectangular bar magnet. The magnet modules are introduced between two disks of a nonmagnetic material at a time in magnet receivers, which are made as form-fit depressions and fixed by screws of a nonmagnetic material.
Since a strong magnetic field is required to induce high voltages in a sensor by the precession of the hydrogen atoms contained in the fluid, strong magnet modules are used. Due to the magnet modules, which are arranged tightly to one another in space in each of the disk magnets, the interaction of the magnetic fields of the individual magnet modules causes major forces between of the magnet modules. These force actions make it very difficult to introduce the individual magnet modules. Often, with introduction under the indicated force actions, unfavorable contact of the magnet modules with the magnet receivers causes peeling of the brittle magnet material. The peeling of the magnet material changes the magnetic field of a magnet module and, thus, adversely influences the homogeneity of the resulting magnetic field in the interior of the magnetization device. Since the voltage induced by the precession of the atomic nuclei into the sensor depends on the Larmor frequency and the latter on the magnetic field strength, peeling of magnet material results in a deterioration of the measurement quality.